The present invention pertains to a safety device for electrically programmable read-only memories such as EPROM memories or EEPROM memories. It pertains more especially to a safety device for electrically programmable read-only memories designed to receive confidential information that cannot be altered by a fraudulent person. Memories of this type are used, for example, in memory cards which are also called CCC cards.
Generally, in memories of the above type, each data storage element or memory cell comprises a floating gate MOS transistor which may be mainly of the FAMOS (floating gate avalanche injection MOS) type or the SAMOS (stacked gate avalanche injection MOS) type. The above MOS transistor may have two states. For an N-channel MOS transistor, in a first state there is no charge trapped in the floating gate. A conduction channel may exist between the source and the drain. The transistor is then conductive and behaves like a closed switch. In a second state, electrons have been trapped in the floating gate. They prevent the creation of a conduction channel in the substrate between the source and the drain. In this case, the transistor is off and behaves like an open switch.
To programme a MOS transistor 1 of the type represented in FIG. 1b, voltages higher than the normal operating voltage are appropriately applied to the control gate 4 and the electrode 2 so that the floating gate 5 absorbs and retains a charge of electrons. This charge of electrons at the floating gate 5 raises the conduction threshold, at the control gate 4 of the transistor, from the minimal theshold voltage V.sub.TO of the non-programmed transistors to a higher threshold voltage V.sub.T1 as shown in FIG. 1a, which gives the graph of the working characteristic of a SAMOS type floating gate transistor.
To read a memory which is thus programmed, a voltage Vcc, lower than the threshold voltage V.sub.T1 of the programmed transistors but higher than the minimum voltage V.sub.TO of the non-programmed transistors, should be applied to the control gate. This reading voltage can be used to detect whether the transistor is on or off. As depicted in FIG. 1b, the transistor 1 generally is linked by the electrode 2 to a bit line 6 which is voltage biased by a generator. The other electrode 3 of the transistor is linked to the ground. The bit line 6 is also linked to a current detector which is not shown. This detector measures the current put through the line by the generator. If the memory cell has not been programmed, the transistor is on and, when the reading voltage Vcc which is greater PG,4 than V.sub.TO is applied, the transistor is put into saturation mode. A drop in current is detected on the detector. In the latter case, when the memory cell has been programmed, charges are trapped in the floating gate of the transistor. The reading voltage Vcc, applied to the control gate is, in this case, in the opposite direction to the potential barrier created in the conduction channel by the charges stored in the floating gate. However, this voltage is then not enough to modify the conduction of the channel and the transistor stays off. As a consequence, the detector at the end of the bit line does not detect the current variation.
When the above memories are used in memory cards, the manufacturer programmes a number of data, some of which must be absolutely non-falsifiable. Information of this type includes for example, the secret code of the user. Now if, for the reading voltage Vcc, a voltage greater than the threshold voltage V.sub.T1 is used, whether or not the memory cells are programmed, a drop in current is detected on the detectors. Consequently, all the memory cells of the secret code are read as if they were not programmed.
To remove this disadvantage, it has been proposed to use, for example, reference bits, namely points that are programmed only once. In this case, if an attempt is made to erase the code, the reference bits are obligatorily erased and the card is invalidated. The card can therefore no longer be used.
The present invention proposes a new method to remove this disadvantage, a method by which the memory card can be used again.